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Today in 2015: New Horizons at Pluto

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New Horizons: Multi-colored rocky planet with mountains, plains and craters.
View larger. | When New Horizons arrived at Pluto in 2015, it found this large heart-shaped feature on the planet’s surface. Pluto’s “heart” measures approximately 1,000 miles (1,600 km) across. It’s a vast plain of nitrogen ice. Image via NASA/ JHUAPL/ SwRI/ The University of Arizona.

July 14, 2015: New Horizons at Pluto

NASA’s New Horizons spacecraft passed distant Pluto on this date, sweeping only about 7,750 miles (12,472 km) above its surface. The fast-moving spacecraft had traveled almost 10 years and three billion miles (five billion km) to reach Pluto. The journey took only about a minute less than what the mission team predicted when the craft launched in January 2006. Once at Pluto, New Horizons “threaded a needle” through a 36-by-57 miles (58 by 92 km) window in space. As an illustration, that’s like a commercial airliner arriving no more off target than the width of a tennis ball.

In addition, New Horizons was the first-ever space mission to view Pluto and its moons (Charon, Nix, Hydra, Styx and Kerberos) up close.

It’ll likely be the only space mission to Pluto in the lifetimes of many of us.

Surprising and amazing

A year after the New Horizons flyby, the mission’s principal investigator Alan Stern of the Southwest Research Institute, Boulder, Colorado, listed the mission’s most surprising and amazing findings:

– The complexity of Pluto and its satellites is far beyond what we expected.
– The degree of current activity on Pluto’s surface and the youth of some surfaces on Pluto is simply astounding.
– Pluto’s atmospheric hazes and lower-than-predicted atmospheric escape rate upends all of the pre-flyby models.
– Charon’s enormous equatorial extensional tectonic belt hints at the freezing of a former water ice ocean inside Charon in the distant past. Other evidence from New Horizons indicates Pluto could well have an internal water-ice ocean today.
– All of Pluto’s moons that can be age-dated by surface craters have the same, ancient age, adding weight to the theory that they were formed together in a single collision between Pluto and another planet in the Kuiper Belt long ago.
– Charon’s dark red polar cap is unprecedented in the solar system and may be the result of atmospheric gases that escaped Pluto and then accreted on Charon’s surface.
– Pluto’s vast 1,000-kilometers-wide (621-miles-wide) heart-shaped nitrogen glacier (informally called Sputnik Planitia) that New Horizons discovered is the largest known glacier in the solar system.
– Pluto shows evidence of vast changes in atmospheric pressure and, possibly, past presence of running or standing liquid volatiles on its surface; something only seen elsewhere on Earth, Mars and Saturn’s moon Titan in our solar system.
– The lack of additional Pluto satellites beyond what was discovered before New Horizons was unexpected.
– Pluto’s atmosphere is blue. Who knew?

New Horizons: 3 people laughing and looking at out-of-sight screen.
New Horizons team members react with joy and amazement. This is the NASA New Horizons Pluto Flyby team viewing the last image before the flyby of Pluto. Image via NASA.
New Horizons: 6 scientists looking at a computer screen react with amazement to the latest image of Pluto.
Science team members react to the latest image of Pluto at the Johns Hopkins University Applied Physics Lab on July 10, 2015. Left to right: Cathy Olkin, Jason Cook, Alan Stern, Will Grundy, Casey Lisse and Carly Howett. Image via Michael Soluri.

Pluto’s heart

Among New Horizons’ most immediate, stunning and visible findings was a bright heart-shaped feature on Pluto. Scientists named it Tombaugh Regio after Clyde Tombaugh. Its nickname is simply The Heart.

New Horizons: On the left, a blurry round ball, on the right a much-crisper image of Pluto with its heart-shaped feature.
Best image of Pluto from Hubble Space Telescope (left) in contrast to a New Horizons image of Pluto (right). Scientists know there is a large bright spot on Pluto, but it takes a spacecraft flyby to reveal that bright spot is Pluto’s iconic Heart.

Ice mountains on Pluto

Another stunning discovery by New Horizons was finding ice mountains on Pluto, with peaks jutting as high as 11,000 feet (3,500 meters) above Pluto’s surface. The mountains lie along Pluto’s equatorial region near the base of The Heart. Scientists think that these mountains likely formed no more than 100 million years ago, making them extremely young in contrast to the 4.56-billion-year age of our solar system. Jeff Moore, a New Horizons imaging team member, said:

This is one of the youngest surfaces we’ve ever seen in the solar system.

Is Pluto geologically active?

In fact, on March 29, 2022, scientists announced that giant ice volcanoes create some of Pluto’s most unusual surface features. Evidence suggests this is from recent activity, geologically speaking. Plus the ice volcanoes may still be erupting today.

Read more about mountains on Pluto

New Horizons: Raised, elongated mountains on right side of orbital image of Pluto.
New Horizons images of a region near Pluto’s equator reveal a range of young mountains rising as high as 11,000 feet (3,400 meters) above the surface. Scientists base this youthful age estimate on the lack of craters in the image above. Like the rest of Pluto, space debris pummeled this region for billions of years. And would have once been heavily cratered, unless recent activity gave the region a facelift, erasing those pockmarks. Image via NASA/ JHUAPL/ SwRI.

Looking back

A year after New Horizons’ Pluto flyby, Hal Weaver, New Horizons project scientist from the Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland, said:

It’s strange to think that only a year ago, we still had no real idea of what the Pluto system was like. But it didn’t take long for us to realize Pluto was something special, and like nothing we ever could have expected. We’ve been astounded by the beauty and complexity of Pluto and its moons and we’re excited about the discoveries still to come.

If you ask these scientists today – years after the flyby – they’d express a lot of the same excitement.

New Horizons: An illuminated edge of Pluto, backlit so that you can see its atmosphere.
New Horizons captured this image 15 minutes after its closest approach to Pluto on July 14, 2015, looking back toward the sun. The image is from a distance of 11,000 miles (17,700 km) to Pluto; the scene is 780 miles (1,255 km) wide. The back-lighting highlights over a dozen layers of haze in Pluto’s tenuous but distended atmosphere. You can see Pluto’s rugged, icy mountains and the flat ice plains extending to Pluto’s horizon. Image via NASA/ JHUAPL/ SwRI.

Where is New Horizons now?

In early 2019, New Horizons encountered a second Kuiper Belt object, officially known as 2014 MU69 and previously nicknamed Ultima Thule. It has since been renamed to Arrokoth. Read more about the Arrokoth/Ultima Thule encounter here.

Team leader Alan Stern has suggested potential for a third flyby of a Kuiper Belt object by New Horizons in the 2020s. But a suitable Kuiper belt object – close enough to the spacecraft’s current trajectory – hasn’t yet been confirmed.

So New Horizons is still out there – still within our solar system – speeding outward.

New Horizons: Dark circular silhouette of backlit Pluto, surrounded by ring of blue haze.
The “blue skies of Pluto” from New Horizons after closest approach, with Pluto backlit by the sun. It is one of the most iconic images of the mission. Image via NASA/ JHUAPL/ SwRI.

Bottom line: The New Horizons spacecraft flew past the Pluto system on July 14, 2015.

Top 10 pics from the Pluto flyby

Charon’s red cap created by “atmospheric surges”

View all images of the Pluto encounter from the LORRI imager on New Horizons

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